I'm not an Aussie but I've spent considerable time there most recently in August. Here is what I can tell you from my observations and what I've read.

The rail systems in Australia were built by the states with the idea of funneling traffic from the country into the major city in that state. So they went their own way with gauges. Queensland went for narrow gauge 3'6", New South Wales went with standard 4'8.5", Victoria built broad gauge 5'3" (1600 mm), South Australia built broad and narrow and Western Australia went for narrow. There were a few oddball narrow gauge systems in Victoria but these are long gone. Remember when the rail building stage was going the states were basically separate colonies of England and not really a country.

When the trans-Australian was built across the desert from South Australia to Western Australia it was built to standard gauge even though it connected to narrow gauge systems on both ends. Starting in the 1960's the federal government started a program to build or convert track to connect all the major cities with standard gauge. This has been done in some cases with new track and some gauge conversion, especially narrowing broad gauge to standard.

As it stands now, Queensland is mostly narrow gauge. The only standard gauge is the line from Brisbane south into NSW. Victoria has converted much broad gauge to standard (except the Melbourne commuter system)and there is a program underway right now to convert more. South Australia has converted broad to standard (except the Adelaide commuter system)and abandoned much narrow gauge. Western Australia converted some narrow to standard and built new standard. Western Australia now has both narrow and standard and this is where most of the dual gauge track is currently. The newer north south line to Darwin was built as standard and the old narrow gauge to Alice Springs was abandoned except for a heritage line on the south end.

The Pilbara iron ore lines are all standard and these lines do not connect or come close to the regular Australian rail system. There is also a short standard gauge line hauling bauxite in islolated far north Queensland.

I should also mention the island state of Tasmania which is narrow gauge.

There is a lot of traffic still on the narrow gauge lines of Queensland. Lots of coal. These are heavy duty in some cases electrified mineral haulers that happen to be narrow gauge. They also move grain and much intermodal and even cattle.